As the exploration scope is enlarged and the exploration depth is increased, drilling in high-temperature and high-pressure reservoir has become a normal state of oil and gas drilling operation. High temperature and high pressure means high risk and high cost, and higher requirements for drilling operation, equipment, and drilling fluid properties.
There are high requirements for the temperature resistance property and density of the drilling fluid used for drilling in high-temperature and high-pressure reservoir. To ensure the temperature resistance property of drilling fluid, usually oil-based drilling fluids, which have an excellent temperature resistance property, are selected for drilling operation in high-temperature and high-pressure wells. However, even an oil-based drilling fluid that has excellent performance is used, the solid content in the system will increase inevitably as the density increases; consequently, on one hand, the solid-solid and solid-liquid friction increases, resulting in increased plastic viscosity of the drilling fluid system; on the other hand, to keep the high solid content in suspension state preventing settlement, the shearing force of the drilling fluid system has to be increased by adding a rheological modifier, etc. However, rheological modifiers commonly used for drilling fluids at present may cause increased apparent viscosity and plastic viscosity of the drilling fluid system, though they can increase the shearing force of the drilling fluid system. Excessive apparent viscosity and plastic viscosity may result in: (1) decreased drilling rate and longer drilling cycle; (2) increased circulation loss and increase drilling pump load; (3) increased equivalent circulation density, and complex downhole conditions such as lost circulation, pressure differential sticking, etc.; ultimately the cost and difficulty of drilling operation are increased.
Therefore, for high-density drilling fluids, usually a viscosity reducer is added, in order to greatly decrease the apparent viscosity and plastic viscosity of the system while maintaining appropriate shearing force, increase drilling rate, reduce drilling difficulties, save drilling cost, prevent downhole accidents, and ensure safe and efficient drilling operation.
At present, viscosity reducers for water-based drilling fluids are available in great varieties and widely applied, while the researches on viscosity reducers for oil-based drilling fluids and relevant products are rare, because oil-based drilling fluids are recognized for long as having characteristics including good rheological property, low viscosity, and easy control, etc. However, as the requirements for the temperature resistance property and density of drilling fluids in high-temperature and high-pressure wells become higher and higher, the apparent viscosity and plastic viscosity of oil-based drilling fluids (even clay-free oil-based drilling fluids) will go far beyond a reasonable range and will be difficult to control if the density of the oil-based drilling fluid is 2 g/cm3 or higher; in addition, usually the applicable temperature doesn't exceed 150° C.